Sizing a subpanel

Let's say I want a subpanel in the garage. It will drive 220V and 110V
outlets. How should I determine the amp on the breaker on the main
panel that would serve the subpanel? That would also determine the
gauge of the wire leading to subpanel, obviously. The simplest way is
the obvious one, add all amp ratings of various outlets, being mindful
that the 110V outlets would be on different legs. Is that the best way
though?
Specifically, what I want is a 30A 220V outlet, a 20A 220V outlet, and
two 20A 110V outlets on two legs. The sum is 70A. Note that it is
unlikely that I will use both 30A and 20A 220V outlets at the same
time. So, this means that I should be able to get away with 50A
service.
Makes sense?
i

You'll use a 100A or 125A "main lug" load center. (70A panels do not
have enough spaces) The panel will cost you about $20 to $30, depending
on the brand.)
The wire size you use is determined by the maximum load you intend to
use all-at-the-same-time and the allowable voltage drop (which is
dependant on the length of the wires, and whether you use copper or
aluminum.) There are tables where you can look up the voltage drop.
Then the breaker in the main panel is determined by the maximum ampacity
of the wire.
My code book is in the garage somewhere, so I will just make some
guesstimates. Your maximum load is probably something like 40A, so you
could use #8 copper cable. But depending on the distance to the main
panel you might wanna use the next larger wire size to reduce the
voltage drop. The ampacity of #6 copper wire is 55A, and you get to
round that up to a 60A breaker. If you use #6 copper SER (service
entrance cable) instead of 6-3 NM cable, I *think* the ampacity is 65A
and you could use a 70A breaker, but don't quote me on that.
If you think you might ever want to run a full-sized arc welder out
there, just go ahead and run #6 copper or #4 aluminum even if something
smaller is adequate for now.
Best regards,
Bob

You need a 4 wire cable. Sub panels must have the neutral and ground
separated.
6-4 SER would be what you looking for.
use this page to calculate the voltage drop for the right size wire.
http://www.electrician.com/vd_calculator.html
A true load calculation is done at the service. I will bet after you add
this load to the service you could use #8 (40 amps) and be fine. Just
in-case you want to save a buck or two

The answer to that last question depends on the code that is enforced in
your area. In Chicago and environs conduit is required for almost
everything. In most other areas you can run SER or any other cable that
is suitable for use indoors.
--
Tom H

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